This first chapter introduces the problem of light (laser beams) transport within strongly (multiple) scattering tissues such as skin, breast, brain, and vessel walls. Basic principles and theoretical descriptions using radiation transfer theory or Monte Carlo (MC) simulation are considered. The propagation of short pulses and photon-density diffusion waves in scattering and absorbing media is analyzed, and the prospects of these methods for tissue spectroscopy and tomography are discussed. Tissue structure and anisotropy, polarization phenomena, optothermal, optoacoustic, and acoustooptical interactions in strongly scattering tissues are described. A discrete-particle model of soft tissue is presented. Fluorescence and inelastic light scattering, including multiphoton fluorescence and vibrational and Raman spectroscopies, are discussed. The design and characterization of tissuelike phantoms for optical diagnostics and light dosimetry are described.
1.1 Propagation of continuous-wave light in tissues
1.1.1 Basic principles, and major scatterers and absorbers
Biological tissues are optically inhomogeneous and absorbing media whose average refractive index is higher than that of air. This is responsible for partial reflection of the radiation at the tissue/air interface (Fresnel reflection), while the remaining part penetrates the tissue. Multiple scattering and absorption are responsible for laser beam broadening and eventual decay as it travels through a tissue, whereas bulk scattering is a major cause of the dispersion of a large fraction of radiation in the backward direction. Therefore, light propagation within a tissue depends on the scattering and absorption properties of its components: cells, cell organelles, and various fiber structures. The size, shape, and density of these structures; their refractive index relative to the tissue ground substance; and the polarization states of the incident light all play important roles in the propagation of light in tissues.
In view of the great diversity and structural complexity of tissues, the development of adequate optical models that account for the scatter and absorption of light is often the most complex step of a study. Two approaches are currently used for tissue modeling. In the framework of the first one, tissue is modeled as a medium with a continuous random spatial distribution of optical parameters; the second one considers tissue as a discrete ensemble of scatterers.